1. Field of the Invention
The present invention relates to a magnetic recording medium provided with a nonmagnetic substrate on which at least an orientation adjusting layer that controls the orientation of a layer just above the nonmagnetic substrate, a vertical magnetic layer with an axis of easy magnetization primarily oriented vertically to the nonmagnetic substrate and a protective layer are disposed, and to a magnetic recording and reproducing device.
2. Description of the Prior Art
A hard disk drive (HDD) that is a type of magnetic recording and reproducing device is currently increased in recording density at a rate of 50% or more a year and it is said that this trend will be continued after this. This is the reason why the development of a magnetic recording head suitable for high-density recording and the development of a magnetic recording medium are now in progress.
At present, the magnetic recording medium mounted on a commercially available magnetic recording and reproducing device is the so-called vertical magnetic recording medium in which the axis of easy magnetization in the magnetic film is primarily oriented vertically. A vertical magnetic recording medium is reduced in the influence of a diamagnetic field in the boundary region between recording bits with high recording densification and therefore, a clear bit boundary is formed to thereby limit an increase in noise. Also, a reduction in recording bit volume along with high recording densification is eventually small, the vertical magnetic recording medium is also resistant to the heat fluctuation effect. In light of this, there is proposed the structure of a medium suitable for the vertical magnetic recording, this structure attracting remarkable attention in recent years.
Recent studies have been directed to the use of a single pole head having the high ability for writing to the vertical magnetic layer to cope with the desire for higher recording densification of a magnetic recording medium. A magnetic recording medium is proposed which is improved in the efficiency of the rate of change of magnetic flux between the single pole head and the magnetic recording medium by disposing a layer made of a soft magnetic material which is called a backing layer between the vertical magnetic layer which is a recording layer and a substrate, to cope with such a single pole head.
However, when a magnetic recording medium simply provided with a backing layer as mentioned above is used, it can be unsatisfied in recording and reproducing characteristics, heat fluctuation resistance and recording resolution during the course of recording and reproduction and a magnetic recording medium superior in these characteristics is desired.
It is essential matter in high recording densification in the future to attain the compatibility between a high S/N ratio enabling an increase in the ratio (S/N ratio) of signal to noise during the course of reproduction and an improvement in resistance to heat fluctuation, which are particularly important as recording and reproducing characteristics. However, these two items are contrary to each other and specifically, if one of these items is improved, the other is deteriorated, giving rise to an important problem concerning a high level of compatibility between these characteristics.
In order to solve such a problem, the publication of JP-A No. 2005-276410 (Patent Document 1) proposes a magnetic recording medium resistant to a reduction in S/N ratio while enjoying such an advantage that synthetic Mrt and PW50 are dropped by joining three magnetic layers by AFC. Specifically, the above Patent Document 1 discloses a magnetic recording medium provided with a substrate, a first lower ferromagnetic layer disposed on the substrate and having a residual magnetization Mr, a thickness t and a product of a residual magnetization and thickness Mrt, a ferromagnetic coupling layer disposed on the first lower ferromagnetic layer, a second lower ferromagnetic layer which has a Mrt value and is disposed on the ferromagnetic coupling layer, an antiferromagnetic coupling layer disposed on the second lower ferromagnetic layer and an upper ferromagnetic layer which is disposed on the antiferromagnetic coupling layer and has a Mrt value larger than sum of the Mrt values of the above first and second lower ferromagnetic layers.
There are no bounds to the request for high recording densification of a magnetic recording medium, and it is demanded of the magnetic recording medium to have higher recording and reproducing characteristics and to be improved in heat fluctuation characteristics. To cope with such a request, it is considered to form the magnetic layer as a multilayer as described in the above Patent Document 1 and also to combine the multilayered magnetic layers by AFC (anti-ferro coupling). However, there is the problem that magnetization inversion is made difficult, and a reduction in OW and an increase in the width of magnetization inversion become a hindrance to the high recording densification of a magnetic recording medium in the future.
It is an object of the present invention to provide a magnetic recording medium and a magnetic recording and reproducing device which have a high S/N ratio, and are superior in heat fluctuation characteristics and in recording characteristics (OW).
The inventors of the present invention have made earnest studies to attain the above objects and as a result, found that a magnetic recording medium which enables easy magnetization inversion, has a higher S/N ratio than usual, and is superior in heat fluctuation characteristics and in recording characteristics (OW) not by coupling the multilayered magnetic layers by AFC but by adjusting ferro-coupling (coupling with magnetization in the same direction) and magnetostatic coupling to proper ranges and also by designing the Ku of the multilayered magnetic layers to be an optimum value can be provided. The inventors of the present invention also have found that, even if materials which hardly generate magnetization inversion are used, a medium can be provided which attains good OW characteristics and magnetization inversion distribution, has a high S/N ratio and is superior in heat fluctuation through the aid of the upper layer and lower layer by using a material having a high Ku for the intermediate layer and by using a material having a lower Ku than those of the intermediate layer in upper and lower layers. Also, the inventors of the present invention have found that when no nonmagnetic layer exist between each magnetic layer of a laminate structure of the magnetic layer, the multilayered magnetic layers are not reversed simultaneously but gradually reversed, leading to a deterioration in OW and wide magnetization inverse distribution of the magnetic layer, which is a cause of the turbulence of bit boundaries, resulting in an increase in noise.